This application relates to processes for reducing nitric oxide, NO, sometimes referred to as NO.sub.x, in combustion zone effluent gas streams.
More particularly, the invention concerns a selective catalytic reduction ("SCR") process which, under limited circumstances, employs pretreatment of a combustion zone effluent gas stream by a selective non-catalytic reduction ("SNCR") process, i.e., only when the NO content of the SCR effluent exceeds a preselected maximum value.
In another respect the invention pertains to an integrated SCR-SCNR NO reduction process which minimizes the cost of chemical reducing agents.
Carbonaceous fuels can be made to burn more completely, with reduced emissions of carbon monoxide and unburned carbon and/or hydrocarbons, when the air/fuel ratio employed causes a high flame temperature. When fossil fuels are used in suspension fired boilers such as large utility boilers, flame temperatures above about 2000.degree. F., to about 3000.degree. F., are generated. Such high temperatures, as well as hot spots of higher temperatures, cause the production of thermal NO, the temperatures being so high that atomic oxygen and nitrogen are formed and chemically combine as nitrogen oxides. Nitrogen oxides, i.e., NO or NO.sub.2 ("NO"), can also be formed as the result of oxidation of nitrogen-containing species in the fuel, such as those found in heavy fuel oil, municipal solid waste and coal. NO derived from nitrogenous compounds contained in the fuel can form even in circulating fluidized bed boilers which operate at temperatures that typically range from 1300.degree. F. to 1700.degree. F.
NO is a troublesome pollutant found in the combustion effluent stream of boilers and other combustion equipment. Nitrogen oxides contribute to tropospheric ozone, a known threat to health, and can undergo a process known as photochemical smog formation, through a series of reactions in the presence of sunlight and hydrocarbons. Moreover, NO is a significant contributor to acid rain, and has been implicated as contributing to the undesirable warming of the atmosphere, commonly referred to as the "greenhouse effect".
Recently, various processes for reducing NO in combustion effluents have been developed. They can generally be segregated into two categories: selective and non-selective. The selective processes include SCR and SNCR processes.
SCR processes involve passing the combustion zone effluent across or through a catalyst bed in the presence of NH.sub.3, to achieve NO reductions as high as 50%-95% or higher. SNCR processes involve introducing NO reducing agents such as NH.sub.3 into the effluent at higher temperatures than SCR processes, to achieve NO reductions of up to 50% or greater.
SCR processes for reducing NO are well known and utilize a variety of catalytic agents. For instance, in European Patent Application WO 210,292, Eichholtz and Weiler disclose the catalytic removal of nitrogen oxides using activated charcoal or activated coke, as a catalyst, with the addition of NH.sub.3. Karo et al., in U.S. Pat. No. 4,138,469, and Henke in U.S. Pat. No. 4,393,031, disclose the catalytic reduction of NO with NH.sub.3, using platinum group metals and/or other metals such as titanium, copper, molybdenum, vanadium, tungsten, or oxides thereof to achieve the desired catalytic reduction.
Another catalytic reduction process is disclosed by Canadian patent No. 1,100,292 to Knight, which discloses the use of a platinum group metal, gold, and/or silver catalyst deposited on a refractory oxide. Mori et al., in U.S. Pat. No. 4,107,272 disclose the catalytic reduction of NO using oxysulfur, sulfate, or sulfite compounds of vanadium, chromium, manganese, iron, copper, and nickel with the addition of NH.sub.3 gas.
In a multi-phase catalytic system, Ginger, in U.S. Pat. No. 4,268,488, discloses treating a NO containing effluent to a first catalyst comprising a copper compound such as copper sulfate and a second catalyst comprising metal combinations such as sulfates of vanadium and iron or tungsten and iron on a carrier, in the presence of NH.sub.3.
SNCR processes were also proposed to remove NO from combustion gas effluent streams by injecting NH.sub.3 or an NH.sub.3 precursor in the presence of oxygen, without using catalysts. For example, such processes are disclosed in U.S. Pat. No. 3,900,554 and in U.S. Pat. Nos. 4,777,024; 5,057,293; and 4,780,289.
In addition, combination SNCR-SCR processes have been proposed, such as the processes disclosed in U.S. Pat. Nos. 4,978,514; 5,139,764; 4,286,467; 4,302,431 and 5,233,934.